In a conventional image scanning apparatus, when the output level of an image sensor has varied due to a change in utilization environment of the apparatus or aging, the tone and color reproducibility of a scanned image becomes unstable and deteriorates. To solve this problem, an image scanning apparatus which can realize stable and high tone and color reproducibility even when the output level of the image sensor has varied due to a change in utilization environment of the apparatus or aging has been proposed (e.g., Japanese Patent Application Laid-Open No. 11-275310).
In the image scanning apparatus described in Japanese Patent Application Laid-Open No. 11-275310, upon scanning a document, a reference white background is scanned for each color light source prior to the beginning of the document scan, the output signal from the image sensor upon scanning the reference white background is converted into digital image data by an A/D converter, and it is checked if the maximum value of that image data falls within a predetermined range. If the maximum value of the image data falls outside the predetermined range, light amount adjustment of the corresponding light source is redone. More specifically, even when the output level of the image sensor has varied due to a change in utilization environment of the apparatus or aging, since the aforementioned process is executed occasionally upon scanning a document image, an image scan with stable and high tone and color reproducibility is realized.
On the other hand, as the image scanning apparatus has higher resolution, a buffer memory for an image process requires a large-capacity memory element. It is advantageous for power savings and a cost reduction to use a DRAM. However, since the access speed of the DRAM is lower than an SRAM, it may bottleneck the scan time. Such low access speed may sacrifice the scan time when a low-resolution scan is made using a high-resolution image sensing element.
To solve this problem a method of executing an image process for assuring an access time to the DRAM after image data is converted into the number of pixels corresponding to the scan resolution has been proposed. In this method, the drive speed of the image sensing element is changed for each scan mode, and a low-resolution scan can be done within a short period of time.
However, in the aforementioned method, since the drive speed of the image sensing element changes depending on the scan mode, fixed pattern noise caused by distortion of an analog signal and the dark current of the image sensing element changes. For this reason, to strictly execute shading correction, shading data must be acquired for each scan mode, and shading correction must be done based on that data.
In case of an image scanning apparatus using a high-resolution (e.g., 1200DPI) image sensing element, scan modes for resolutions of, e.g., 75DPI, 150DPI, 300DPI, 600DPI, and 1200DPI should be prepared by hardware in both color and gray modes. For this purpose, calibration data and shading data for each scan mode must be prepared. However, since the number of types of scan modes is large, several minutes to about 10 minutes may be required to collectively acquire these data in the first scan.
On the other hand, calibration data and shading data may be acquired for each scan. However, a longer time is required for each scan as the resolution increases.